Recently, laser diodes have come to be widely used in a variety of types of electrical equipment, for example, laser printers, optical disk apparatuses, fiber-optic communication apparatuses, and mobile phones, because of their compact size, low power requirements, and long life.
FIG. 1 is a schematic block diagram of a generally known laser diode drive circuit. The laser diode drive circuit generally includes a drive current control circuit 10 and a power supply circuit 20. The power supply circuit 20 is connected to a Vdd power supply source. The output terminal Vout of the power supply circuit 20 is connected to an anode of the laser diode LD. The power supply circuit 20 supplies power to the laser diode LD by generating an output voltage Vo at the output terminal Vout.
The drive current control circuit 10 is connected to a cathode of the laser diode LD through an output terminal thereof. The drive current control circuit 10 receives an image data signal (video signal) DATA and switches the laser diode LD on and off according to the image data signal DATA. The drive current control circuit 10 supplies a predetermined constant current to the laser diode LD to light the laser diode LD.
There is a large fluctuation in drive voltage Vop of the laser diode LD due to variation arising during the manufacturing process of the laser diode LD. For example, the drive voltage Vop of the blue laser diode may vary from 3.5 v to 5 v or more. Accordingly, the output voltage Vo of the power supply circuit 20 must be set to a voltage greater than the contemplated drive voltage to be used in consideration of these fluctuations in the drive voltage Vop of the laser diode LD. More specifically, it is necessary that the output voltage Vo is set to a larger voltage than the maximum drive voltage Vopmax (hereinafter simply “maximum drive voltage”).
When a drive current is supplied to the laser diode, a LD terminal voltage Vdr is generated at the LD terminal. The LD terminal voltage Vdr is the voltage at the output terminal of the drive current control circuit 10 to which the cathode of the laser diode is connected. The LD terminal voltage Vdr is a voltage obtained by subtracting the Vop that is the drive voltage of the laser diode from the output voltage Vo of the power supply circuit 20. Consequently, the output voltage Vo of the power supply circuit 20 is set to a voltage higher than a voltage obtained by adding the LD output terminal voltage Vdr to the maximum drive voltage Vopmax.
However, when the output voltage Vo of the power supply circuit 20 is set to a voltage which is the sum of the maximum drive voltage Vopmax and the LD output terminal voltage Vdr as described above, a difference between the actual drive voltage Vop and the maximum drive voltage Vopmax may be consumed as unnecessarily, resulting in wasted power consumption. For example, when a drive current is set to 0.1 A and the maximum drive voltage Vopmax is set to 6 v, and the actual drive voltage of the laser diode LD is 4 v, the difference between the actual drive voltage and the maximum drive voltage Vopmax, i.e., 2 v, is added to the LD output terminal voltage Vdr. Accordingly, since the drive current is 0.1 A, the drive current control circuit 10 consumes 0.2 w of power needlessly.
In addition, recent remarkable developments in low power technology has reduced power supply voltages Vdd significantly, and the power supply voltage for the most apparatuses is now 3.3 v or 5 v. However, such voltage is not enough to drive blue laser diodes, which need a high drive voltage Vop. Accordingly, it is necessary to boost the power supply voltage with the power supply circuit 20. For this reason, it is necessary that the power supply circuit 20 employ a boost-type switching regulator. If the supply circuit 20 employs the boost-type switching regulator, the power supply circuit 20 consumes power even while the laser diode LD is not being driven. Therefore, it is necessary to delay start up of the power supply circuit 20 until just before the laser diode LD is driven, so as to reduce power consumption and save power.
However, if the power supply circuit 20 starts up slowly, it is necessary to make the power supply circuit 20 operate much earlier than a time the laser diode LD is driven, which causes a problem for high-speed operation of the laser diode LD. Consequently, it is necessary to raise the output voltage Vo of the power supply circuit 20 quickly up to a necessary voltage to light the laser diode LD.